Rotogravure printing process

ABSTRACT

There is provided an improved process for printing by rotogravure printing means which is characterized by the use of an overcoating of a styrene-acrylonitrile copolymer which can be applied over a printed image at high speeds encountered in such printing process without adversely affecting the equipment.

United States Patent [72] Inventors PaulR.Brown [54] ROTOGRAVURE PRINTING PROCESS 8 Claims, No Drawings [52] U.S.Cl 117/12, 117/15,117/45, 117/155 UA, 117/161 UN [51] lnt.Cl B44d 1/16, 844d H09 [50] Field of Search 117/161 UN,45,12, 15; 115/155 U [56] References Clted UNITED STATES PATENTS 2,597,087 4/1951 Cowgill 117/161 UlN 3,239,365 3/1966 Petty 1 17/45 3,479,197 10/1969 Mork 117/45 OTHER REFERENCES Styrene Polymers and Copolymers, in Modern Plastics Encyclopedia, ed. by Hiram McCann, N.Y., Vol. 40, No. 1A pp. 288- 289 (Sept., 1962) Primary Examiner-Alfred L. Leavitt Assistant Examiner-M. F. Esposito Attorne yMcNenny, Farrington, Peame and Gordon ABSTRACT: There is provided an improved process for printing by rotogravure printing means which is characterized by the use of an overcoating of a styrene-acrylonitrile copolymer which can be applied over a printed image at high speeds encountered in such printing process without adversely affecting the equipment.

ROTOGRAVURE PRINTING PROCESS This invention relates as indicated to an improved process for rotogravure printing. It is characterized by the application of a clear lacquer composed of a relatively low molecular weight styrene-acrylonitrile copolymer as an overcoat to the printed material which acts as a protective coating and an appearance-improving coating. The invention has particular utility in the field of printing labels for packaged foodstuffs, and particularly labels for metallic containers.

Such labels are customarily printed upon a label stock which is a very inexpensive paper stock, and consequently difficulty is experienced in the holdout" of the ink compositions used for the printing of such labels. To improve the appearance as well as the durability of such labels for handling including resistance to scuffing and marring, it has been customary to apply a clear lacquer of a synthetic resin such as a styrenated alkyd of relatively high molecular weight, which upon drying provided an attractive glossy surface and which rendered the label resistant to scuffing, marring, and accidental tearing. A particular problem with many of these overcoating materials has been that their drying mechanism was not only one of solvent removal, but also of oxidation. As a consequence of the required prolonged drying time, the printing equipment was unable to handle such material at normal speeds. Either the speed had to be cut down or the equipment modified to allow sufficient time and to provide sufficient temperature for the curing of the resinous film. These materials provided all the desired characteristics, and the holdout or resistance to soaking into the paper web was achieved through the high molecular weight of the resinous material.

It has now been found most unexpectedly that polymers of styrene and acrylonitrile containing from 90 to 50 parts by weight styrene and, correspondingly, to 50 parts by weight of acrylonitrile which are mass or solution polymerized to a relatively low molecular weight, i.e., 1,000 to 15,000, provide a clear glossy resinous film of good holdout, flexibility and resistance to handling, and which is at the same time readily accommodated by rotogravure printing equipment at linear paper speeds up to as high as 800 feet per minute. These resins are particularly useful in that there is a minimum of fouling of the equipment and the equipment is readily cleaned with the solvent system for the resin, or ketone solvents. Suitable lowboiling, i.e., below 200 C., solvents include benzene, toluene, methyl ethyl ketone, ketones in general, methylene chloride, ethyl acetate, o-dichlorobenzene, tetrahydrofuran, dimethyl formamide, dioxane, cyclohexanone, acetone, butyrolactone, or mixtures of two or more of the foregoing; e.g., a 1:1 mixture of toluene and methyl ethyl ketone; a toluene methyl ethyl ketone ethyl alcohol mixture, 4.5:4.5:1; etc. Those solvents having boiling points below about 100 C. are preferred. Styrene/acrylonitrile resins useful herein are commercially available.

Although styrene/acrylonitrile copolymers have found considerable utility as injection-molding materials, their brittleness, inability to retain plasticizers, slight tensile elongation and related properties have militated against their use in coatings, so that films processed therefrom have heretofore found no practical applications.

Briefly stated, therefore, the present invention is in a new use for styrene-acrylonitrile polymers of relatively low molecular weight in a low-boiling solvent as an overcoat for a printed image in a rotogravure printing process.

The polymer is thermoplastic and contains no materials which dry by oxidation. Because of these factors, high press speeds are possible. The polymer dries by solvent evaporation alone. Once the solvents are removed in the drying operation, there will be no blocking of the coated stock at a temperature of about 130 F. and a pressure of about 2 psi. The coating is completely rewettable in its own solvent and accordingly cleaning of equipment is made very easy.

The coating compositions of the present invention may be used in supplement or replacement of general purpose rotogravure nitrocellulose lacquers. They offer advantages in economy, gloss, window light stability, scuff and mar resistance. The lacquers of the present invention are equal to or better than commercially available lacquers in respect of adhesion, flexibility, resistance to the contents and resistance to water spotting.

The lacquers of the present invention are conveniently produced by a single solution polymerization procedure. Mass polymerization may also be used but is less desirable in view of the need to reduce the solids content for application. Styrene and acrylonitrile monomers are introduced into a steamjacketed kettle in the desired mole ratio along with catalyst and a reflux solvent. The mass is heated to reflux temperature and the batch held at a slight reflux until complete conversion of the monomers. A suitable catalyst is a free radical catalyst such as benzoyl peroxide or cumene hydroperoxide. A suitable refluxing solvent is methyl ethyl ketone. Aromatic hydrocarbons alone or in combination with alcohols may be used as refluxing solvents, and particularly satisfactory results have been attained with toluene as the reflux solvent. In a specific example wherein styrene and acrylonitrile monomers were mixed in a molar ratio of 2:1 (or approximately :20 on a weight basis), the catalyst was benzoyl peroxide in an amount of 1 percent by weight, and the solvent was methyl ethyl ketone.

To prepare the solution polymer, the reaction mass is maintained at a temperature of about 170 F. for a period of 2 hours and thereafter at a temperature of about 230 F. for a period of about 9 hours. The reaction is carried out under an inert gas blanket such as nitrogen or carbon dioxide. At 55 percent NVM (nonvolatile matter) this product is water white and has a Gardner-Holdt viscosity of Z6. The resin is soluble in aromatic hydrocarbons, ketones, and esters. It is insoluble in alcohols and aliphatic hydrocarbon solvents. These polymers tolerate alcohol as a diluent in solution inaromatics or esters. The polymer per se has a ball and ring softening point of 315 F. In like manner other copolymers of styrene and acrylonitrile may be produced and used for the purposes hereof. The styrene may be replaced in whole or in part by vinyl toluene, and may include up to 5 parts by weight in parts of resin of alpha-methyl styrene.

The kettle base in this particular example had a viscosity of 10 stokes and a total solids content of 50 percent. This base may be thinned either with toluene, acetone, methyl ethyl ketone, or any other suitable solvent. Marproofing materials such as polymethyl silicone may be added (0.5 percent by weight of resin solids) and solvent added to a viscosity of 30 seconds (No. 2 Zahn cup) and a total solids content of 30 percent by weight. ln order to prepare the material for use on the press, a further reduction to a total of about 20 to 25 percent solids with a solvent such as methyl ethyl ketone or toluene is desired. This is considerably higher solids content than is normally used for these purposes.

The lacquers of the present invention are applied after printing the label and at a rate on the dry basis of from 0.5 pounds per ream (3,000 square feet) to 2 lb. per ream. The press speeds may be varied from to 800 feet per minute and the drier temperatures for effecting rapid dry of the lacquers of the present invention may be in the range of from to 275 F. The dry time at 180 F. is on the order of 2 seconds. Hot air at 180 F. may be used, or a steam-heated drying roll. The limitations of machine speed imposed by the initial label printing itself will determine the final line speed because of the wide latitude aflorded by the overcoating composition. Except for the overcoating step the printing operation is unchanged from prior rotogravure practice.

There has thus been provided an improved rotogravure printing process in that there has been substituted for the conventional nitrocellulose clear label lacquers or styrenated alkyd lacquers, a thermoplastic styrene-acrylonitrile copolymer which may be applied by conventional rotogravure presses at speeds up to 800 linear feet per minute without necessitating special drying equipment or operating at lower speeds to achieve proper drying, and which material may be applied at relatively high solids contents on the order of from 20 to 25 percent NVM and relatively high viscosities on the order of from 15 to 25 seconds No. 2 Zahn to provide a dry coating on a rotogravure printed paper surface which is characterized by gloss and by scufi" and mar resistance.

These lacquers are economical to use and require no modification of existing rotogravure equipment and require no limitation on the linear speed at which the paper may be processed through the printing apparatus. It normally would have been expected that normal molecular weight polymers of the molecular size of the resinous materials hereof would not have satisfactory holdout" on label stock paper and would not be able to be dried at high solids contents in a period of time to allow high linear paper speeds. These lacquers also provide a further advantage in that there is minimum buildup, and whatever buildup there is may be removed readily from the apparatus by the same solvent system in which the resin was initially dissolved.

What is claimed is:

1. In a rotogravure printing process the improvement which comprises applying a clear coating of a solution of a low molecular weight solution polymerized copolymer of styrene and acrylonitrile having a molecular weight of from 100 to 15,000 in a solvent having a boiling point below 200 C. as an overcoat for a printed image, and removing the solvent by evaporation to provide a dry protective coating for said printed image.

2. A process in accordance with claim 1 in which the lowboiling solvent has a boiling point below about C.

3. A process in accordance with claim 1 in which the styrene and acrylonitrile are copolymerized in a weight ratio of from 90 to 50 parts by weight of styrene and, correspondingly, from 10 to 50 parts by weight acrylonitrilc.

4. A process in accordance with claim I in which the copolymer is one formed from 80 parts by weight of styrene and 20 parts by weight acrylonitrile.

5. A process in accordance with claim 1 in which the solvent is removed at a temperature of from to 220 F.

6. A process in accordance with claim 1 in which the solids content of the copolymer in the solution is from 20 to 25 percent.

7. A process in accordance with claim 6 in which the solvent comprises a ketone.

8. A process in accordance with claim 7 in which the ketone is methyl ethyl ketone. 

2. A process in accordance with claim 1 in which the low-boiling solvent has a boiling point below about 100* C.
 3. A process in accordance with claim 1 in which the styrene and acrylonitrile are copolymerized in a weight ratio of from 90 to 50 parts by weight of styrene and, correspondingly, from 10 to 50 parts by weight acrylonitrile.
 4. A process in accordance with claim 1 in which the copolyMer is one formed from 80 parts by weight of styrene and 20 parts by weight acrylonitrile.
 5. A process in accordance with claim 1 in which the solvent is removed at a temperature of from 170* to 220* F.
 6. A process in accordance with claim 1 in which the solids content of the copolymer in the solution is from 20 to 25 percent.
 7. A process in accordance with claim 6 in which the solvent comprises a ketone.
 8. A process in accordance with claim 7 in which the ketone is methyl ethyl ketone. 